1. Field of the Invention
The present invention relates to a power steering device for vehicles, and more particularly to a variable-steering-ratio steering device having a steering ratio which is variable relative to angular movement of a steering wheel.
2. Description of Relevant Art
A steering ratio on power steering devices employing hydraulic pressure for powered steering is determined from the standpoint of the steering handling during stopping or low-speed travel of a vehicle, as well as the straight-moving stability during high-speed travel. However, it is impossible to have a fixed steering ratio which successfully meets both of such requirements. While the vehicle is at rest or moving at a low-speed, it is desirable to have as small a steering ratio as possible so as to turn dirigible wheels through a relatively large angle in response to a slight turning of the steering wheel. While the vehicle is running at a high speed, the steering ratio should be large enough to prevent over-steering so as to ensure stability of the vehicle.
In an effort to meet such requirements, there has been proposed and employed a variable-gear-ratio steering device for varying the gear ratio of a steering gear in response to the turning of the steering wheel in such a manner that the gear ratio is relatively large when turning of the steering wheel is slight, and becomes progressively smaller as the turning becomes greater.
Such known devices, however, are complicated in structure and require precise machining during manufacture because the variable gear ratio is determined by the variable-pitch rack of a rack-and-pinion steering gear. Furthermore, it has been impossible to provide a required range of variable gear ratio while at the same time permitting the driver to steer the vehicle smoothly.
The present invention provides a steering device having a vehicle speed sensor driven in proportion to the road surface resistance acting on the vehicle wheel, and a reaction force mechanism cooperating with the vehicle speed sensor, for giving the driver a steering reactive force which becomes heavier with increasing vehicle speed.